2) (TODO) Using inferred validations on Virtus attributes, please see
Aequitas::Inferred. Note that not all validations that are provided via
validation methods, are also available as autovalidation options. If they
are available, they're functionally equivalent though.

See Aequitas::Macros to learn about the complete collection of validation
rules available.

Validating

Aequitas validations may be manually evaluated against a resource using the
`#valid?` method, which will return true if the resource is valid, and
false if it is invalid.

Working with Validation Errors

If an instance fails one or more validation rules, Aequitas::Violation
instances will populate the Aequitas::ViolationSet object that is available
through the #errors method.

For example:

my_account = Account.new(:name => "Jose")
if my_account.valid?
# my_account is valid and has been saved
else
my_account.errors.each do |e|
puts e
end
end

See Aequitas::ViolationSet for all you can do with the #errors method.

Contextual Validation

Aequitas also provide a means of grouping your validations into contexts.
This enables you to run different sets of validations when you need it. For
example, an instance may require separate validation rules depending on its
state: publishing, exporting, importing and so on.

Each context causes different set of validations to be triggered. If you
don't specify a context using :when, :on or :group options (they are
all aliases and do the same thing), default context name is :default. When
you do model.valid? (without specifying context explicitly), again,
:default context is used. One validation can be used in two, three or five
contexts if you like:

class Book
include Virtus
include Aequitas
attribute :id, Serial
attribute :name, String
attribute :agreed_title, String
attribute :finished_toc, Boolean
# used in all contexts, including default
validates_presence_of :name, :when => [:default, :sending_to_print]
validates_presence_of :agreed_title, :when => [:sending_to_print]
validates_with_block :toc, :when => [:sending_to_print] do
if self.finished_toc
[true]
else
[false, "TOC must be finalized before you send a book to print"]
end
end
end

In the example above, name is validated for presence in both :default
context and :sending_to_print context, while TOC related block validation
and title presence validation only take place in :sending_to_print context.